Okay, I think I know what you're talking about. Lava in 18w15a has a wierd lighting glitch that seems to decrease the saturation. I don't think this is intentional and I don't think either of us should have to worry about it.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.